1. Field of the Invention
The present invention resides in a technical field relating to semiconductor devices. In particular, the present invention resides in a technical field relating to semiconductor devices that use a semiconductor element such as a transistor.
2. Description of the Related Art
Development of semiconductor devices having light-emitting elements has been advanced in recent years. In addition to the existing advantages of liquid crystal display devices, the semiconductor devices also have characteristics such as fast response speed, superior dynamic display, and a wide angle of view. The semiconductor devices are attracting attention as flat panel displays for the next generation of mobile devices, which will be capable of utilizing dynamic contents.
A semiconductor device having light-emitting elements has a plurality of pixels, which each have a light-emitting element and at least two transistors. The transistor that is connected in series with the light-emitting element in the pixel (hereinafter written as a driving transistor) plays a role in controlling light emitted from the light-emitting element. The light-emitting element has a structure having a first electrode, a second electrode, and a light-emitting layer sandwiched between the first electrode and the second electrode. One electrode connected to a source electrode or a drain electrode of the driving TFT is referred to as a pixel electrode, and the other electrode is referred to as an opposing electrode.
Static electricity builds up, developing due to friction, contact, and the like with air and objects such as conductors, semiconductors, and insulators. Electrostatic discharge occurs if an object is strongly charged. If this phenomenon develops in a free node such as an input terminal of the semiconductor device, then minute semiconductor elements manufactured on a substrate will degrade or be destroyed. This is referred to as electrostatic discharge damage.
In order to prevent electrostatic discharge damage, a circuit formed on a substrate (hereinafter written as an internal circuit 64) is connected to an externally attached IC (hereinafter written as an external circuit 61) through a protecting means (also referred to as a protection circuit) 63 and an FPC 62 as shown in FIG. 15. The protecting means 63 detects a voltage and a current supplied from the external circuit 61 to the internal circuit 64, and controls the values of the voltage and the current in order to prevent damage to the internal circuit 64 during abnormal operation.
When manufacturing a semiconductor device having a light-emitting element, TFTs are first manufactured over a substrate, and light-emitting elements are manufactured thereafter. More specifically, TFTs are first manufactured over a substrate, and then wirings are manufactured so as to be electrically connected to source regions and drain regions of the TFTs. Pixel electrodes of the light-emitting elements are then manufactured so as to be electrically connected to the wirings. The pixel electrodes are in an exposed state when manufactured up to this point, and therefore static electricity tends to build up in the pixel electrodes. In particular, the pixel electrodes become antennas during manufacturing processes involving charged particles, such as dry etching and electron beam evaporation, and electrostatic discharge is easily induced. Sudden discharge of electric charge that has built up in the pixel electrodes leads to degradation or destruction of the semiconductor elements connected to the pixel electrodes.